Petroleum is commonly recovered from subterranean, petroleum-containing formations or reservoirs in which it has accumulated, in a primary recovery phase comprising pumping or permitting the petroleum to flow to the surface of the earth through wells drilled into and in fluid communication with the subterranean formation. Petroleum is recovered only if certain conditions are found. There must be an adequately high concentration of petroleum in the formation, and there must be adequate permeability or interconnected flow channels throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the petroleum. When the subterranean formation has natural energy such as an underlying active water drive, solution gas or a high pressure gas cap above the petroleum, this natural energy source is utilized in the primary petroleum recovery phase. When this natural energy source is depleted, or in the instance of those formations which do not originally contain sufficient natural energy to facilitate primary recovery, some form of supplemental recovery process must be applied to the reservoir. Supplemental recovery is frequently referred to as secondary recovery, although it in fact may be primary, secondary or tertiary in sequence of employment.
The petroleum process comprising injecting water into the formation for the purpose of displacing petroleum toward the production wells, commonly referred to as waterflooding, is the most economical and widely practiced form of supplemental recovery. It is well recognized by persons skilled in the art that water does not displace petroleum efficiently, however, since water and oil are immiscible and also because there is a high interfacial tension between water and oil at reservoir conditions. Persons skilled in the art of oil recovery have recognized this limitation of waterflooding and many additives have been described in the prior art for decreasing the interfacial tension between the injection water and formation petroleum. Petroleum sulfonates have been disclosed in many references, and are quite economical and effective in low salinity formations, but cannot be used alone in petroleum formations containing water having salinity greater than about 30,000 parts per million total dissolved solids.
Many prior art references describe the use of more complex synthetic surfactants, or combinations of synthetic surfactants and petroleum sulfonate or other organic sulfonates, for oil recovery from oil formations containing relatively high salinity water. For example, U.S. Pat. Nos. 3,811,504; 3,811,505; and 3,811,507 describe surfactant waterflooding methods using two and three component systems involving anionic and nonionic surfactant mixtures. U.S. Pat. Nos. 3,348,611 and 3,508,612 describes the use of a sulfated and ethoxylated alcohol in combination with petroleum sulfonate, for surfactant waterflooding oil recovery. U.S. Pat. No. 3,500,923 describes the use of a highly saline, turbid dispersion containing an ethoxylated and sulfated surfactant. U.S. Pat. Nos. 3,827,497; 3,890,239; 4,018,278; and 4,088,189 describe surfactant waterflooding oil recovery processes employing a sulfonated and ethoxylated surfactant which is especially useful in high temperature formations containing high salinity water.
While the foregoing surfactant systems have yielded encouraging laboratory test results, field use has been less than entirely satisfactory for several reasons. Some of the more complex synthetic surfactants exhibit unexpected fluid phase instability, which requires use of additional additives to stabilize the fluid and prevent separation into separate phases which degrade oil recovery efficiency. Interactions have been observed between certain ethoxylated surfactants and at least some hydrophilic polymers employed in mobility buffer fluids commonly injected immediately after surfactant fluids in state-of-the-art waterflooding oil recovery processes. Moreover, while particularly surfactants may be found which can be used in very high salinity environments, particularly the sulfated or sulfonated and ethoxylated surfactants, the degree of ethoxylation and other variables in the surfactant molecule are very critical to the effectiveness of the surfactant at a particular salinity, and a particular surfactant identified for a specific salinity loses efficiency rapidly on contacting water of either higher or lower salinity. It is not unusual to experience variations in formation water salinity from one portion of the formation to another.
In view of the foregoing discussion, it can be appreciated that there is a significant need for an improved surfactant waterflooding oil recovery process suitable for use in high temperature and/or high salinity formations which overcome at least certain of the foregoing drawbacks of prior art methods. It is an object of the present invention to overcome at least some of these drawbacks by employing the preferred embodiments of the fluid and process to be described more fully hereinafter below.